Quantcast Section II. Electromagnetic Interference

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
been installed. Ungrounded conductors on construc-
age between ungrounded hardware and pole
assembly grounded parts. Of these three causes, the
tion in progress, as well as ungrounded hardware
third is the most common and the most difficult to
on the pole assembly, should be treated with the
locate and will be treated in the most detail.
same care and precautions as energized conductors.
(1) To locate noise from the first two causes,
The minimum spacing (given earlier) should be
items of line equipment should be examined for the
maintained between all ungrounded pole assem-
following defects:
blies.
(a) Loose hot-line clamps.
(4) One type of leakage noise, which is common
(b) Corroded fuse ferrules in cutout boxes.
and puzzling, comes from staples on the pole
(c) Defective surge arresters (particularly
ground. In some cases staples are driven too near
the gapped valve type).
the crossarm through bolt. If the gap is small
(d) Defective insulators and transformer
enough between the staple points and the through
bushings.
bolt, a leakage discharge occurs. Capacitance be-
(e) Defects in the internal insulation of a
tween the through bolt and the phase wire, and its
associated tie wires, develops an electrostatic
(f) Loose connections in a neutral circuit.
charge during each half cycle of the supply fre-
(g) Loose ties on insulators and neutral
quency to produce a square wave with 120-hertz
brackets.
frequency. The electrical interference from this
(h) Covered tie wire used on a bare conduc-
wave is generally most noticeable at radio fre-
tor.
quencies near 800-kilohertz, although harmonics
(i) Loose pole-line hardware.
present from the 120-hertz square wave shape can
(j) Insufficient spacing between grounded
cause interference up to the l00-megahertz region
and ungrounded parts of pole assembly. Spacing
of the radio frequency spectrum. Line attenuation
should be 2 inches (51 millimeters) minimum for 7.2
generally reduces the higher megahertz frequency
kilovolts and 8 inches (204 millimeters) minimum
harmonics, so that they do not spread as far as
for 14.4 kilovolts.
harmonics in the broadcast band. Taps tune the
noise signal, so that some noise frequencies are
(2) Noise from electrostatic leakage is most
damped out while others are accentuated. The dis-
commonly found on a multigrounded neutral type of
charge gap length influences the striking voltage of
circuit and is the most difficult to trace. Noise which
the arc and the width of the square wave, which is
originates on the primary, due to leaky insulators
related to the fundamental frequency of the noise
and the like, generally dies out within a few spans
signal. If the noise wave is examined on an oscillo-
and is easily found. Noise which arises from electro-
scope, it appears as a small spike on the tip of each
static leakage and gets into the neutral circuit may,
half cycle of the 60-hertz power wave.
under some conditions, be detectable for 10 miles
(a) Leakage noise occurs in a similar manner
(16 kilometers) on each side of the source. This
noise is caused by arcing due to insufficient spacing
when ungrounded crossarm braces are too close to
ground wire or grounded guys, and when neutral
between some item of ungrounded hardware, within
ground wires are too close to phase-wire pins on
the electrostatic field of the primary conductors,
crossarms. Fuse-cutout brackets'and surge-arrester
and some part of the grounded pole assembly.
brackets generate leakage noise if they are too close
(3) It is important to recognize that any piece
to grounded pole members. Weatherproof wire used
of pole-line hardware, which is near an energized
for the pole ground lead may cause trouble if the
conductor, may pick up enough electrostatic charge
staples break the insulation but do not make solid
to spill over a small gap. Cases of interference have
contact with the copper ground wire conductor.
been traced to long lengths of barbed wire fencing
(b) In older construction, where a neutral
running for some distance under a power line before
discharging to ground. In one case the entire fence
conductor is carried on a metal bracket instead of on
acted as an antenna and transferred the noise back
an insulator, EM1 may be created and be difficult to
into the primary. This noise was detectable for 5
locate. A loose tie on one of these brackets, even
miles (8 kilometers) from the source on an automo-
though the bracket and its bolt are not grounded at
bile receiver. For construction in progress, conduc-
the pole in question, causes the development of a
tors should be tied in on insulators and all wires
noise signal which will travel over three or four
should be grounded to the system neutral at several
spans. A multiplicity of these noise signals can blan-
points to prevent possible interference and also to
ket a wide area. Tightening these ties eliminates
the noise for a short period until conductor vibra-
serve as a safety precaution. This precaution also
applies to telephone system construction which has
tion loosens the ties again. The permanent cure for
not reached the stage where drainage coils have
this type of noise is changing insulators brackets to
16-2





 


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